1. Field of the Invention
This invention relates to a focus detecting system which enables the integration period required for a focus detecting operation to be made as short as possible by means of a photosensor device constituted by photoelectric conversion elements, such as static induction transistors (hereinafter referred to simply as "SIT(s)"), from which stored photoelectric charges can be read out non-destructively.
2 Description of the Related Art
In general, when focus detection is to be performed by means of an automatic focus control apparatus incorporated in a camera or the like, it is necessary to obtain signals corresponding to the contrast of an object. However, if the signal level is too small, problems such as the influences of noise or a quantizing error due to A/D conversion become serious. On the other hand, if the signal level is so large as to exceed the dynamic range of A/D conversion, it will be impossible to obtain correct detection signals. Accordingly, in order to effect focus detection with accuracy, it is necessary to control the integration period required for a focus detecting operation so that output signals of optimum magnitude can be obtained from a photosensor device irrespective of the intensity of the brightness of an object.
However, if these problems are to be solved, various other problems will occur in focus detecting apparatus of the conventional type employing a photoelectric conversion device such as a CCD (charge-coupled device) as a photosensor device. Such a focus detecting apparatus adopts a destructive readout method in which once a stored signal is read out for the purpose of confirming the level of an output signal, the corresponding stored photoelectric charge is lost. It is, therefore, impossible to control the integration period directly on the basis of the output signal level of the photoelectric conversion device.
To overcome this problem, in Japanese Patent Laid-open No. 64,711/1982, U.S. Pat. No. 4,410,258, British Patent No. 2,082,010, West German Patent No. 3,131,053 and so forth, it has been proposed to provide an image sensor in which a separate photoelectric conversion device such as a photodiode for monitoring the quantity of light is arranged in the vicinity of a focus-detecting photoelectric conversion element array such as an CCD or the like so that the integration period of the photoelectric conversion element array can be controlled by undirectly inferring the signal output level of the focus-detecting photoelectric conversion element array on the basis of the output level of the monitoring photoelectric conversion device.
Furthermore, Japanese Patent Laid-open No. 140,409/1984 proposes a method in which if an object is of low brightness, focus detection is performed by amplifying a photosensor output to apparently increase its integration period. In this focus detecting system, if an object is of such low brightness that an integration operation for providing signals required for focus detection is not completed within a predetermined period, with the result that the amount of charge stored is less than a predetermined value, the photosensor output signal is amplified at an amplification ratio not smaller than 1 to thereby enable focus detection. In this fashion, this conventional method intends to prevent deterioration in response by restricting the integration period to not longer than a predetermined time period, as well as to prevent deterioration in detection accuracy by amplifying the photosensor output signal.
In addition, Japanese Patent Laid-open No. 26,016/1986 discloses the following focus detecting method. Specifically, if an object is of low brightness, the integration operation is stopped when the elapsed time of the integration period reaches a predetermined period T.sub.1 and the amplification ratio of a photosensor output signal is determined from the output level of a monitoring photoelectric conversion device that was provided at the time that the integration was stopped. Based on the output signal thus amplified, focus determination is performed once. If, in this focus determination, no in-focus state is obtained and it is determined that focus detection is impossible, the integration period is extended to T.sub.2 (&gt;T.sub.1) and integration is carried out again.
However, in the above-described conventional focus detecting apparatus which is arranged so that the integration period is determined, for convenience' sake, on the basis of whether the output of the monitoring photoelectric conversion device, which is separately disposed in the vicinity of the photoelectric conversion device such as a CCD, has reached a predetermined reference value, it is not deemed that an integration period of rational length can be determined according to the state of each individual object. For instance, even in the case of an object which has low brightness but contrast high enough to enable focus detection thereof, this focus detecting apparatus continues integration until the output of the monitoring photoelectric conversion device reaches the predetermined value. As a result, a time period longer than it would originally take is consumed until an in-focus state is detected.
If the brightness of an object is high, this disadvantage does not provide any serious influence over the total period required for a focus detecting operation, since the integration period is originally short in the case of focus detection as to a high-brightness subject. However, if an object has such low brightness that a long integration period is required, such disadvantage becomes serious, so that blurring and deterioration in response may be caused.
Furthermore, the conventional focus detecting system, which is arranged to amplify its photosensor output when an object is of low brightness, involves the non-rationality that the amplification ratio of the photosensor output level must be determined according to the monitor output level rather than the photosensor output level itself. In addition, in the focus detecting method disclosed in the above Japanese Patent Laid-open No. 26,016/1986, once the amplification ratio of the photosensor output signal is determined on the basis of the monitor output level which was reached after the passage of a predetermined integration period, the photosensor output signal is amplified at this amplification ratio. If no in-focus state is detected with the output signal thus amplified, the integration period is extended and integration is again performed. This repetition of integration requires a time-consuming and complicated operation.